The central nervous system undergoes extraordinary proliferative growth during embryonic and early postnatal life. It is traditionally held that once the full complement of neurons is produced, there is no further proliferation of CNS neurons. However, a small number of new neurons and glia have been demonstrated to be generated throughout life. Recent studies suggest that neuronal and glial cells may arise from the ependymal layer lining the ventricles of the brain and central canal of the spinal cord. Little is known regarding the responsiveness of these stem cells to neurotoxic injury. We hypothesize that neural stem cells located in the ependyma respond to neurotoxic stimuli by proliferation and differentiation of progeny into neural cells, and that these newly produced cells play a role in the brain's response to neurotoxic injury. To test this hypothesis, we propose to address three specific questions: (1) Do ependymal cells proliferate in response to toxicant-induced loss of neurons from neighboring regions of the brain? (2) Do progeny of ependymal cells migrate to areas of neurotoxic damage and become integrated into the brain? (3) Do neural stem cells of immature brains respond to neurotoxic injury in the same manner as neural stem cells in adults? Three model neurotoxicants will be used to compare the responsiveness of ependymal cells following direct injury (urea), damage to hippocampal neurons adjacent to the ventricular system (trimethyltin) and remote injury to the olfactory system (zinc sulfate). Ependymal cells will be double- labelled by intraventricular injection of DiI and with BrdU administered in drinking water, and their movement and fate determined by immunofluorescence and immunocytochemistry. The progeny from ependymal cell replication will be characterized using antibodies to specific markers for neurons, astrocytes, or oligodendroglia. Juvenile and adult animals will be examined in parallel to determine whether the response is age-dependent. The goal of the proposed study is to characterize the contributions of ependymal stem cells to cellular responses to neurotoxicants. Results may elucidate age-dependent differences in brain adaptability to injury, and may suggest strategies for therapeutic repair of neurotoxic damage.